We know floods are coming, Heimer said. “It’s just a matter of when.

— John Heimer

When John Heimer started working as a raft guide on the Boise River in 1965 the cheapest property in town was on River Street.

"The river was a polluted mess from discharges from cattle operations, discharges from butcherings," he said. "We just didn't think that the river was anything. At the time it was the second most polluted river in the state of Idaho."

A former Idaho Rivers United board member, Heimer is one of the premier local experts on the Boise River. In addition to working as an outfitter on the river for 20 years, he is retired from the Idaho Department of Fish and Game, where he worked as a biologist. Heimer spoke to a crowd of two dozen visitors during a brown-bag lunch at the IRU offices April 18.

When the Clean Water Act was passed in 1972, things started to change for the Boise River.

"The river was cleaned up, and all of a sudden people started floating it," he said. "It's amazing what we can do if we put our minds to it." The power and influence of the law combined with action by people who cared made a clear difference, he said.

Heimer walked his crowd through the river's history and focused on how development has encroached on the river's banks. People are drawn to it because it flows clear and cold, but that same development has narrowed the channel, which he estimated to be about half its pre-European capacity.

That's not a problem most of the time. This year, at the time of Heimer's presentation, the river was flowing at more than 8,000 cubic feet per second for more than 25 days in a row, and the weather hadn't really started to warm up much at that point.

The U.S. Army Corps of Engineers said there are only 10 percent odds this spring that the Boise River will run over 10,000 cfs, but the ongoing accumulation of snow in the high country isn't helping the odds. Heimer said geologist estimate that the Boise River flowed at 100,000 cfs or more in 1862 before there were any dams on the system. The next highest estimate is 35,500 cfs in 1896. Either flow would create widespread destruction in Boise today.

The first of three dams, Arrowrock, was built in 1915 and at the time was the largest dam in the world. Anderson Ranch Dam on the South Fork of the Boise was finished in 1950, and Lucky Peak Dam was finished in 1955. Collectively, the three reservoirs store just over a million acre feet of water.

The three dams do help with lower magnitude flooding, but there's no way these dams could have contained the 1896 or 1862 floods, both of which would have quickly overpowered the system. What's more, this isn't a problem you can build your way out of, and dams often cause a false sense of security. People then build in flood plains and flood-prone areas. This is exactly what happened in the Treasure Valley.

"We know floods are coming," Heimer said. "It's just a matter of when."

A section of the Boise River from Barber Park to Ann Morrison Park is now one of the most floated sections of river in the nation, and nearly the entire urban river corridor from Eagle up stream to Boise is a prized greenbelt, which Heimer called "a tribute" to future generations.

"It's up to us to protect the river for future generations," he said. "It's a golden opportunity for all, and for something we can pass down and say: look what we did."